Is Biochar a Solution to Climate Change? Maybe, Maybe Not.

A new paper published earlier this week in the scientific journal PLoSOne calls into question whether we know enough about biochar to use it as an important strategy to mitigate climate change. The article, two of whose co-authors formerly worked here at UCS, did a systematic review of the scientific literature on biochar through 2011, and found 311 relevant papers.

But even with all this research, a key question remains unanswered: How long does biochar persist in the soil?

Biochar looks like and is chemically similar to charcoal. SOURCE: marcus5spot, Flickr.com

What is biochar? And how could it reduce CO2 emissions?

Basically, biochar is what you get by using a low-oxygen process called pyrolysis to burn biomass, creating a carbon-rich and supposedly very stable product similar to charcoal. The more volatile parts of the biomass (from the carbon-hydrogen and carbon-oxygen bonds, for you chemists in the crowd) are lost but almost all the carbon remains. Thus, you don’t produce large amounts of the greenhouse gas carbon dioxide, as you do in burning biomass with normal oxygen levels. The product looks like and indeed is chemically very similar to charcoal – that is, mostly just plain carbon in its reduced form.

But, rather than firing up the grill and barbecuing with it, you put it in the soil. This increases the soil’s organic matter content and is thought to have benefits in terms of soil fertility, water-holding capacity, and crop growth. But besides those possible advantages, doing this also keeps the biochar’s carbon from being oxidized, and thus returning to the atmosphere as CO2, for some period of time

There’s the rub. “Some period of time” – but how long is that? Are we talking a few years or many centuries?

This is critical for whether making biochar and adding it to soil is really a useful way to reduce CO2 emissions. If it remains stable in the soil for a long period of time, then (other things being equal, and of course they never are), the answer is yes, because you’ve delayed the process by which biomass decomposes and returns to the atmosphere. This means that effectively, you’ve reduced greenhouse gas emissions.

But if the period of time is short, then you haven’t truly reduced emissions, just shifted their timing slightly. So the question of how long biochar is stable in the soil is critical for whether it’s an effective climate mitigation tool.

Biochar, bagged for use by urban gardeners in New England. SOURCE: njeremijenko, Flickr.com

What does the science say?

What did Gurwick, Moore, Kelly, and Elias find in their review? What is the answer to this key question? Unfortunately, it’s that we don’t know enough to say.

Even though they found 311 peer-reviewed research articles on biochar, the overwhelming majority were studies in the laboratory, not in soils under natural conditions. Of those 311, only seven actually estimated the decomposition rates of biochar in situ. And in those seven, the estimates of the “mean residence time” of biochar in the soil ranged from 8.3 to 3,624 years.

That’s a pretty big range, to say the least. If biochar remains stable in soils for thousands of years, it can be an effective climate mitigation solution. But if we’re talking about less than a decade, it can’t.

This means that scientifically, a critical question remains unanswered. We have hundreds of published studies, but only a handful focus in the issue of stability in the field, and those few don’t agree.

So, the conclusion is, in the words with which practically every published paper seems to end, “we need more research.” Often that seems to be a throw-away line, or (viewed more cynically) a plea for more funding. But with this paper, we have the scientific evidence that it really is the case.

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Thanks to UCS for directing attention to biochar. I heard Dr. Johannes Lehmann of Cornell (mentioned in earlier comments) just this week at a Biochar Symposium held in Amherst, Mass. and in his address he recommended speaking of biochars and biochar systems in the plural, because there is great variability in their efficacy for soil applications and in the methods for their production. There is an abundance of research being done in these variables to tailor them to targeted uses, even though there is limited mainstream support or commercial investment. The biochar community also shows a great awareness of the need for self-regulation through adoption of testing and labeling regimens for the physical and chemical characteristics of biochars.

One of Dr. Lehmann’s final comments was a caution against the mistake of championing one solution to the expense of others. I believe biochar can be an excellent component of a strategy of reversing the worldwide depletion of carbon-based soil organic matter, and it shows its greatest value in quickly restoring the most depleted of soils to support fertility and rebuild local hydrology. This kind of restoration is crucial to effectively cool the planet during the coming century while we hope to draw down atmospheric CO2 levels.

Doug Boucher

Brian,

Thanks for your comment. Dr. Lehmann is one of the leading researchers in the field of biochar, and the PLoSOne review article included several of the papers on which he is one of the co-authors. Your point about the diversity of biochar(s) is a good one, and this diversity may be one of the factors in the large variability in estimates of how stable biochar is in soils.

I applaud the UCS for sharing this important topic with its readership. However, I feel that it is important that your readers understand that close to 2 years of the most relevant research on the topic (2011-present) has been omitted. The topic of Biochar has received an explosive amount of attention, and not less than 3 new papers are published in the literature each week. Before judging the efficacy, I urge readers to read the proposed guidelines and associated tests that would be required to certify Biochar products for carbon negative offset. http://www.biochar-international.org/protocol. A 100 year halflofe threshold limit for carbon stability in soil is proposed and its import to understand that such a threshold would need to be tested for and certified by an appropriate lab before any Biochar would qualify as an offset in any registry system. The Biochar industry is working to operate with full transparency and the efforts of The Clomate Trust, Prasino Group, and International Biochar Initiative further serve to reinforce this sentiment. It is only through full due dillengence and truth testing that we stand a chance of removing net greenhouse gasses from the atmosphere with implications to climate change.

Doug Boucher

Morgan,

Thanks very much for your comment. As you point out, the PLoSOne paper used as their cutoff date the end of 2011, so there are other articles published since then that were not included in their analysis. On the other, they found that the great majority of papers that they reviewed, did not test the key assumption — how stable is biochar’s carbon under field conditions? Such studies take a long time to do, but they’re critical to biochar’s effectiveness as a climate solution.

Do you know of 2012 or 2013 papers that provide field data on this question? If so, what do they show?